How are Brightest Cluster Galaxies different from normal galaxies?

At the centers of most galaxy groups and clusters there is often one galaxy
which is brighter and more massive than the other cluster galaxies
(Fig. 1). Scientists from Max Planck Institute for Astrophysics and
from Edinburgh have set out to determine how
the environment at the center of galaxy clusters affects the properties of
these Brightest Cluster Galaxies (BCGs).

Fig. 1:
The center of the galaxy cluster Abell 2244. Many galaxies are visible, but
the one at the center is by far the brightest.

Fig. 2:
A "gallery" of BCGs. The BCGs are sorted according to the mass of the host
cluster (from the smallest systems in the top left corner to the most massive
ones in the lower right corner), and every 18th BCG in our sample is shown.

In clusters of galaxies, various physical processes play a role: Clusters are
gravitationally dominated by dark matter
but also contain vast quantities of hot gas
(
Research Highlight May 2004).
This hot gas cools by emitting X-ray
radiation, decreasing its temperature and allowing more gas to flow to the
center (so-called "cooling flows",
Research Highlight November 2005).
The galaxy
at the center of a galaxy cluster sits at the center of the dark matter halo,
where also the gas density is highest. Furthermore, these galaxies assembled
via many mergers of mostly cluster galaxies. The goal of our investigation was
to find out whether any of these processes cause BCGs to be notably different
from normal galaxies.

For a sample of 625 galaxy groups and clusters from the Sloan Digital Sky
Survey (SDSS;
Research Highlight April 2002)
we have selected the brightest galaxy in
the densest, central part of the cluster (Fig. 2). From a catalog of 200000
SDSS galaxies, we chose for each BCG a control galaxy of similar stellar mass,
redshift, and color (the color of a galaxy reflects the average age of its
stars). Any systematic difference between the two samples can be attributed to
the environment at the cluster center.

We find that BCGs are larger than non-BCGs, and have higher velocity
dispersions (a measure of the speed of the stars that make up the
galaxy). These findings imply a higher dark matter fraction in BCGs,
which can be viewed as a direct consequence of their location at the center of
a dark matter halo.

Since most BCGs are elliptical galaxies, we can test whether they follow the
scaling relations which apply to other ellipticals. One of these is the
Faber-Jackson relation, which relates the galaxy luminosity and velocity
dispersion. While the non-BCG control sample lies on the same
relation as elliptical galaxies in general, for BCGs the relation is
significantly different. This is a confirmation of theoretical studies, which
predict that the Faber-Jackson relation is determined by the eccentricity of
merger orbits, i.e. the merger remnants of galaxy mergers which occur on
highly elliptical orbits lie on a different Faber-Jackson relation than those
taking place on more circular orbits. Since clusters are located at the
intersections of the "cosmic web", the BCGs are expected to form from mergers
along the filaments of the matter distribution, and thus mostly elliptical
merger orbits.

Finally, we find that BCGs are more likely to host a radio-loud active
galactic nucleus ("AGN"). This is particularly interesting, since it
has been suggested that these AGNs stop the cooling flows in clusters
(
Research Highlight November 2005).

Although the properties of BCGs are not exceedingly different from normal
galaxies, we can trace the influences of three processes typical of the
cluster center: the dark matter halo alters the structural properties, the
merging history is reflected in different scaling relations, and the hot gas
causes BCGs to be more likely to host radio-loud AGN.